Amines and heterocyclic amines are widely used in various industries, including chemical industry, pharmaceutical industry, and so on. They are present in waste waters drained by industrial factories, farms, drug manufacturing enterprises, food producing enterprises, and so on. Low concentrations of amines are poisonous to humans and animals. Because all the above samples contain higher concentrations of other co-existing organic compounds, detecting the concentration of amines selectively, rapidly, and sensitively is important for environmental protection, industry monitoring and food quality assurance.
For the detection of nitrogen-containing compounds, the nitrogen-phosphorus detector (NPD) or thermal-ionization detector (TID) is the most sensitive detector at present. But the NPD/TID responds to almost all types of nitrogen-containing compounds and cannot respond selectively to amines. Moreover, the NPD/TID is not very sensitive to tertiary amines, secondary amines, and heterocyclic amines. Its minimum detectable amount towards those amines is only about 10−10 g, and the repeatability is not very good. Besides, hydrogen is necessary as the burning gas when the NPD works, which is a potential safety risk.
The principle of the surface ionization detector (SID) is surface ionization. Molecular beam or atoms (or molecules) in the vapor collides with a hot solid surface, and parts of atoms (or molecules) are ionized into positive or negative ions when they are thermally desorbed from the solid surface, as shown in FIG. 1.
For some positive ion produced by ionization, wherein
                              N          =                                    N              0                        +                          N              ±                                      ;                              α            +                    =                                                    N                +                                            N                0                                      =                                                                                g                    +                                                        g                    0                                                  ⁢                                  exp                  ⁡                                      (                                                                  Φ                        -                                                  I                          ⁢                                                                                                          ⁢                          P                                                                                            k                        ⁢                                                                                                  ⁢                        T                                                              )                                                              =                              A                ⁢                                                                  ⁢                                  exp                  ⁡                                      (                                                                  Φ                        -                                                  I                          ⁢                                                                                                          ⁢                          P                                                                                            k                        ⁢                                                                                                  ⁢                        T                                                              )                                                                                                          (        1        )            
N+ is the number of the thermally desorbed positive ion per unit area and unit time;
N0 is the number of the thermally desorbed neutral atom (or molecule) per unit area and unit time;
Φ is the work function of the metal; if the metal is oxidized, Φ increases;
IP is the ionization energy of the atom or molecule;
g+ and g0 are the statistical weight factor of the positive ion and that of the neutral atom or molecule, respectively;
k is the Boltzmann constant;
T is the surface temperature of the metal emitter.
Compounds that can be surface ionized include: 1) IA group metal atoms with low ionization energies, such as sodium, potassium, strontium atom, and so on; 2) oxides of transition metals, rare-earth metals and uranium, inorganic compounds containing sulfur or halogens; 3) organic compounds containing nitrogen or oxygen, such as triethylamine, trimethylhydrazine, aniline, phenols, some amino acids, acetic acid, formic acid, and so on. Among the organic compounds, the surface ionization efficiencies of amines, hydrazines and their derivatives are very high. The ionization efficiency of tertiary amines even reaches 0.2˜0.5. Studies of mass spectrometry show that the types of the ions produced by surface ionization of organic amines are very few, mainly including (M−H)+, (M−Alk)+, (M+H)+, (M−H−2nH)+, (M−Alk−2nH)+, and so on. On collision with hot metal surfaces, most organic compounds first dissociate into radicals with low ionization energies and are then surface ionized. Assuming an organic compound dissociates into radicals s, r, p, q, and so on. For a radical s,is(T)=enYs(T)·βs(T)  (2)
n is the total number of the organic compound colliding with the metal surface;
e is the electron charge in an electron;
T is the surface temperature of the metal emitter;
Ys(T) is the yield of the organic compound dissociating into radical s;
βs(T) is the ionization efficiency of radical s. According to equation (1),
                                          β            s                    ⁡                      (            T            )                          =                                            α              s                                      1              +                              α                s                                              =                      1                          1              +                                                A                  s                                      -                    1                                                  ⁢                                  exp                  ⁡                                      (                                                                                            I                          ⁢                                                                                                          ⁢                                                      P                            s                                                                          -                        Φ                                                                    K                        ⁢                                                                                                  ⁢                        T                                                              )                                                                                                          (        3        )            
Supposing the produced total current is Itotal,Itotal=is(T)+ip(T)+iq(T)+ir(T)=en(Ys(T)·βs(T)+Yp(T)·βp(T)+Yq(T)·βq(T)+Yr(T)·βr(T))  (4)
According to equation (4), for an organic compound that happens to be surface ionized, values of Ys(T), Yp(T), Yq(T), Yr(T), βs(T), βp(T), βq(T), and βr(T) are constants once the structure of detector, the surface temperature of the metal emitter, the electrical field strength, the makeup gas flow, etc. are fixed. Thus the total current produced by surface ionization is proportional to the mole amount of the organic compound. It is the theoretical basis for quantitatively determination of organic compounds by SID.
Usually, Φ−IP<0. According to equation (2) and (3), the higher the work function Φ and the temperature are, the higher the ionization efficiency is. In order to improve the ionization efficiency, metals with high work functions should be used as the emitter, such as platinum, iridium, molybdenum, and so on. Besides, metal surfaces should be probably oxidized in order to increase the work function; and metals should be heated to stay at a high surface temperature.
According to equation (2) and (3), the surface ionization efficiency β is relative to the ionization energy Φ of the organic compound. The smaller the ionization energy is, the more easily surface is ionized. If the difference in ionization energies of two compounds is 1 eV, their ionization efficiencies can differ maximally by 105 times. Among the organic compounds, amines, hydrazines and their derivatives with the lowest ionization energies are the most easily to be surface ionized. Therefore, the SID can detect them with high selectivity.
Shimadzu Corporation developed the commercialized SID in 1986 (Patent No.: CN86103355, JP61264256, EP26223, WO8606836, U.S. Pat. No. 5,014,009, DE3686162). The detector can detect amines selectively. Its response to tertiary amines is 105˜106 times higher than that to ketones. It has almost no responds to hydrocarbons; and its selectivity is very high. To tertiary amines, the sensitivity of the SID is 10˜100 times higher than that of the NPD. To secondary amines and primary amines, the sensitivity of the SID is not as high as that of the NPD, but higher than that of the flame ionization detector (FID). The SID only requires the carrier and the makeup gas, and hydrogen is not necessary. Its operations are simple, and the use is safe.
However, there are some disadvantages in the SID commercialized by Shimadzu as in the following: 1) The electrical current is directly applied on the coiled platinum wire to heat it. Thus there must be enough space between adjacent wire loops to guarantee insulation. Moreover, the coiled platinum wire works also as the emitter, therefore, the surface area of the emitter is relatively small, and the sensitivity is not high. 2) Only the nozzle part is kept hot, and most of the metal housing and the stainless steel collecting electrode are located in a place with relatively low temperatures. Since the alkalinity of amines is relatively high, they are apt to be adsorbed by metals, causing significant peak tailing, and cannot detect tertiary amines of less than 10−11 g/s.
Therefore, if the adsorption of amines by metal materials in the detector is maximally reduced or avoided, SID can detect tertiary amines of less than 10−12 g/s, which will be one of the most sensitive and selective detectors to amines and hydrazines.